The present invention relates to a method for connecting a flexible printed circuit board to another printed circuit board, and relates to the connected printed circuit boards.
A method for connecting a flexible printed circuit board, which includes thermoplastic resin, to a motherboard is disclosed in JP-A-2001-111209. In the method in the publication, a flexible printed circuit board 500 includes an insulating board 50 and a plurality of conductive patterns 51, which are located on the insulating board 50. A mother board 550 includes an insulating board 55 and a plurality of conductive patterns 56, which are located on the board 55. The flexible printed circuit board 500 and the mother board 550 are overlapped such that each predetermined end portion of the conductive patterns 51 and each corresponding end portion of the conductive patterns 56 confront each other with solder paste 54 in-between, as shown in FIG. 7. In FIG. 7, the conductive patterns 51, 56 are respectively covered with solder resists 52, 57. However, the solder resists 52, 57 may be omitted.
Then, the overlapped portion is heat pressed by a heat-pressing tool 60 at a temperature higher than the glass transition temperature of the resin included in the insulating board 50. With the heat pressing, each conductive pattern 51 and each corresponding conductive pattern 56 are electrically connected by the solder paste 54, and simultaneously the resin of the insulating board 50 is softened to bond the insulating board 50 to the insulating board 55 of the motherboard 550.
However, according to the above method, the insulating board 50 is extremely deformed at the area pressed by the heat-pressing tool 60 and in the proximity of the area, as shown in FIGS. 8A, 8B, and 8C. Specifically, the insulating board 50 is thinned at the heat pressed area and thickened in the proximity of the heat pressed area because a swollen portion 58 is formed by the resin extruded from the heat pressed area. As a result, when bending deformation is repeatedly applied to of the connected boards 500, 550, the stress due to the bending deformation is concentrated in the vicinity of the boundary between the swollen portion 58 and the thin heat pressed area because the swollen portion 58 is difficult to bend. As a result, the conductive patterns 51 readily break at the stress-concentrated region.